1. Field of the Invention
In general, the present invention relates to a solid-state image sensing device and its driving method. In particular, the present invention relates to a solid-state image sensing device having a so-called transversal-shutter structure wherein electric charge stored in a sensor is discarded away to an electric-charge exhausting unit provided in the transversal direction of the sensor and relates to a method for driving the solid-state image sensing device.
2. Description of Related Art
A typical conventional configuration of a solid-state image sensing device having a transversal-shutter structure such as a CCD linear sensor is shown in FIG. 10 whereas FIG. 11 shows potentials at a cross-sectional plane X-Y shown in FIG. 10. As shown in FIG. 10, a sensor array 102 comprises a plurality of sensor units 101 which each serve as a photoelectric conversion element. The sensor units 101 composing the sensor array 102 are arranged to form a straight line. An electric-charge transferring unit 104 is provided on one side of the sensor array 102. Read gates 103 are provided between the sensor array 102 and the electric-charge transferring unit 104 which is used for transferring electric charge read out from each of the sensor units 101 by the read gate 103. Electric-charge exhaust drains 106 are provided on the other side of the sensor array 102. Electric-charge exhausting gates 105 are provided between the sensor array 102 and the electric-charge exhaust drains 106 which are each used for discarding away electric charge read out from each of the sensor units 101 by the electric-charge exhausting gate 105. The electric charge transferred by the electric-charge transferring unit 104 is converted into a voltage by a charge-voltage converting unit 107 created at one end of the electric-charge transferring unit 104. A voltage signal resulting from the charge-to-voltage conversion is supplied to an output terminal 109 through a buffer 108, which output terminal 109 then transmits the voltage signal to a signal processing system not shown in the figure.
In a normal operation to read out electric charge in the configuration described above, a read gate pulse .o slashed.ROG is applied to a read gate electrode 111 of the read gate 103. The read gate pulse .o slashed.ROG changes a potential shown in FIG. 11 beneath the read gate electrode 111 from a shallow level RL to a deep level RH, reading out electric charge from each of the sensor units 101. Then, by applying typically two pulses .o slashed.H1 and .o slashed.H2 having phases different from each other to each of transfer gate electrodes 112 which are arranged in the transfer direction of the electric-charge transferring unit 104, electric charge is read out from a portion below the read gate electrode 111 and then transferred by the electric-charge transferring unit 104. In order to change the store time of electric charge in any of the sensor units 101, on the other hand, a shutter pulse .o slashed.SG is applied to a shutter gate electrode 113 of the electric-charge exhausting gate 105 in a period other than an electric-charge storing period. The shutter pulse .o slashed.SG changes a potential shown in FIG. 11 beneath the shutter gate electrode 113 from a shallow level SL to a deep level SH, discarding away electric charge generated in the sensor unit 101 during the period other than the electric-charge storing period to the electric-charge exhaust drain 106.
By the way, in a CCD linear sensor used as a sensor for reading bar codes or as an automatic focusing (AF) sensor of a camera, a sensor structure lengthened in the electric-charge reading direction is adopted in order to increase the optical sensitivity. In the case of a CCD linear sensor used as a sensor for reading bar codes, for example, the dimensions of an opening for a picture element are of the order of 14 .mu.m.times.200 .mu.m. If the transversal shutter structure described above is adopted in a CCD linear sensor having such a sensor structure, however, two problems are encountered. One of the problems is a read residual image due to poor reading which is in turn attributed to the long sensor structure. The other problem is a shutter residual image due to imperfectness of the shutter operation. The effect of the read residual image, one of the two problems, can be reduced by setting the read time at a large value. However, this technique imposes restrictions on conditions under which the CCD linear sensor is used.
As a countermeasure that can be taken for solving the problem as long as the structure of the sensor is concerned, a staircase-like change in potential is provided in the sensor unit 101. As shown in a potential diagram of FIG. 12, the staircase-like change takes place in the read direction of the sensor unit 101. The potential gradient having such a staircase-like shape in the sensor unit 101 makes it easy to read out electric charge therefrom, providing a technique which is considered to be capable of solving the problem due to poor reading which is in turn caused by the long sensor structure. However, even though the problem caused by poor reading can be solved by adopting this technique, the transversal shutter structure described earlier by referring to FIG. 10 can not be adopted because of the staircase-like change in potential the sensor unit. In other words, the shutter structure with a configuration wherein the electric-charge transferring unit 104 is provided on one side of the sensor array 102 while the electric-charge exhaust drain 106 is provided on the other side of the sensor array 102 can not be adopted.
On the other hand, a solid-state image sensing device having a shutter configuration wherein an electric-charge transferring unit and an electric-charge exhausting unit are provided on the same side of the sensor array is also known. An example of such a solid-state image sensing device is disclosed in Japanese Patent Laid-open No. Sho59-74667. In its shutter configuration, store electrodes are provided on the same side of the array of sensors (each serving as a light-sensitive picture element) as the electric-charge transferring unit (or a CCD register). One store electrode is provided for each picture element. The store electrode is used for storing electric charge generated by the picture element at the lower portion of the store electrode. An electric-charge exhausting unit is provided beside each store electrode for exhausting the electric charge generated at the lower portion of the store electrode. In the case of the conventional solid-state image sensing device having this shutter structure, however, a configuration wherein an electric-charge exhausting unit is provided for each picture element is adopted. Accordingly, a space for the electric-charge exhausting unit provided for each picture element in addition to the store electrode is required. As a result, such a shutter structure gives rise to a hindrance to the reduction of the size of the solid-state image sensing device.